Unraveling the Nature of Magnetism of the $5d^4$ Double Perovskite ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$

We report electron spin resonance (ESR) spectroscopy results on the double perovskite ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$. On general grounds, this material is expected to be nonmagnetic due to the strong coupling of the spin and orbital momenta of ${\mathrm{Ir}}^{5+}$ ($5d^4$) ions. However, controversial experimental reports on either strong antiferromagnetism with static order at low temperatures or just a weakly paramagnetic behavior have triggered a discussion on the breakdown of the generally accepted scenario of the strongly spin-orbit coupled ground states in the $5d^4$ iridates and the emergence of a novel exotic magnetic state. Our data evidence that the magnetism of the studied material is solely due to a few percent of ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ir}}^{6+}$ magnetic defects while the regular ${\mathrm{Ir}}^{5+}$ sites remain nonmagnetic. Remarkably, the defect ${\mathrm{Ir}}^{6+}$ species manifest magnetic correlations in the ESR spectra at $T\lesssim 20\mathrm{K}$, suggesting a long-range character of superexchange in the double perovskites as proposed by recent theories.

Figure 1

ESR spectra (field derivatives of absorption) at the $X$-band frequency $\nu =9.56\mathrm{GHz}$ at three selected temperatures. The lines at $\sim 340$ and $\sim 360\mathrm{mT}$ and a triplet structure centered around $\sim 460\mathrm{mT}$ are labeled as $g_{\mathrm{left}}$, $g_{\mathrm{mid}}$, and $g_{\text{right}}$, respectively, with the spin values assigned to each line.

Figure 2

Frequency $\nu$ vs resonance field $H_{\mathrm{res}}$ dependence of the peaks in the ESR spectrum (data points). Solid lines are fits to the relation $h\nu =g{\mu }_B{\mu }_0{H}_{\mathrm{res}}$ yielding the $g$-factor values as indicated in the plot. The insets show spectra at two selected frequencies. The spectrum at 9.56 GHz was obtained by integration of the absorption derivative spectrum (cf. Fig. 1). Arrows in the upper inset indicate the expected positions of the satellites of the $g_{\text{right}}$ peak in the spectrum at 82.18 GHz which are resolved at 9.5 GHz (lower inset).

Figure 3

$T$ dependence of the inverse ESR intensity $1/I^{\mathrm{ESR}}$ at $\nu =9.56\mathrm{GHz}$ and its comparison with the static bulk susceptibility $\chi$: (a) total ESR intensity (diamonds, left scale), bulk susceptibility (circles, right scale), and its Curie-Weiss fit ${\chi }^{-1}=[{\chi }_0+C/(T-{\theta }_{\mathrm{CW}}){]}^{-1}$ with the antiferromagnetic Curie-Weiss temperature ${\theta }_{\mathrm{CW}}=-16\mathrm{K}$ and the Curie constant $C=0.0294{\mathrm{cm}}^3\mathrm{K}/\mathrm{mol}$ corresponding to the effective magnetic moment ${\mu }_{\mathrm{eff}}=0.48{\mu }_B/\mathrm{Ir}$ (solid line, right scale); (b) intensities of individual lines $1/I_i^{\mathrm{ESR}}$. For better comparison, the data are scaled as indicated in the legend.

Figure 4

$T$ dependence of the width $\mathrm{\Delta }H$ of the ESR signals $g_{\mathrm{left}}$, $g_{\mathrm{mid}}$, and $g_{\text{right}}$ (main peak) at $\nu =9.56\mathrm{GHz}$.